Rheological fluid devices, such as magnetorheological (MR) and electrorheological (ER) fluid clutches, polishing devices, and damping mechanisms, such as shock absorbers, utilize the increased yield stress exhibited by a rheological fluid when exposed to a field (e.g., a magnetic or electrical field) in order to control the interface of two mechanical members, such as the coupling of a stator and a rotor that are relatively rotatable. The transmission of torsional or axial forces between the members is controlled by turning the field on and off. Rheological fluids include magnetorheological and electrorheological fluids, and are sometimes referral to as controllable rheological fluids or field-responsive fluids.
For example, an MR fluid is a suspension of magnetizable particles in a carrier fluid. When exposed to a sufficiently high magnetic field, and when the MR fluid is under shear flow, the magnetizable particles will line up in columns of higher concentration, aligned with the direction of applied magnetic field. It has been observed that forming a grooved surface on one or both mechanical members of a torque-transmitting device, with the grooved surface in contact with an MR fluid in an annular space between the members, enhances the stress transmitted through the MR fluid relative to that of a surface without grooves. The grooved surface creates drag in the fluid, inhibiting slip at the boundary of the fluid and the member or members having the grooved surface. The drag is thus beneficial for intended operation in “clutch on” (i.e., magnetic field applied) conditions.